Variable mask field exposure

ABSTRACT

A method of fabricating integrated circuits according to a first design by imaging a first layer on a substrate using a first mask having a block of first patterns in common with a second design, but without any other patterns of the first or second designs and imaging a second layer on the substrate using a second mask having a block of second patterns unique to the first design and at least one third layer pattern. The block of first patterns is repeatedly exposed in a first grid and the block of second patterns is repeatedly exposed in a second grid, each without overlap in the corresponding layer. The grids are aligned such that the integrated circuits and test structures in scribe lines between the integrated circuits are properly formed on the substrate. The first patterns can be for large fields and the second patterns can be for small fields.

FIELD

This invention relates to the field of integrated circuit fabrication.More particularly, this invention relates to reducing the costs andincreasing the efficiencies of the photolithographic processes usedduring integrated circuit fabrication.

BACKGROUND

Integrated circuits are formed of a variety of different layers that arebuilt up, one upon another, where each layer is patterned so as toconstruct structures that perform a variety of different functions.Typically, integrated circuits have been extremely customized. By thisit is meant that a given integrated circuit design, even though it mayuse transistors, resistors, capacitors and other structures that arecommon with other designs, tends to have those structures disposed inunique locations and as parts of uniquely designed circuits. Thus, themask sets for an integrated circuit design have typically been usableonly for a single integrated circuit design, with different designsrequiring entirely separate mask sets.

Masks have typically been fabricated according to one of two differentdesigns. In a first mask design, the mask contains the image patternsrequired for a single given pattern layer of a single given integratedcircuit design. Typically, the image patterns on the mask cover as muchof the surface of the mask as is practical. The benefit of this maskdesign is that the image patterns on the mask tend to cover a largeportion of, or even all of, the surface area of a substrate to bepatterned with the mask. This is typically referred to as a wide fieldexposure mode, because the aperture of the exposure tool is opened wideto expose all of the images on the mask. Thus, this mask design providesfor a relatively high throughput of substrates through the imagingprocess that employs such a mask. However, mask costs tend to be quitehigh when using this type of mask, because a separate mask is requiredfor each masking layer of each integrated circuit design.

In the second mask design, the mask contains different blocks of imagepatterns required for several given pattern layers of a single givenintegrated circuit design. Typically, the blocks of image patterns onthe mask are limited in size, so that more blocks can be fit onto thesurface of the mask. The benefit of this mask design is that mask costsare reduced, because fewer masks are required to complete the integratedcircuit design. This is because the image patterns for several differentlayers of the integrated circuit design can be placed onto a singlemask. Thus, this mask design provides for reduced mask cost. This istypically referred to as a narrow field exposure mode, because theaperture of the exposure tool is restricted so as to only expose thedesired portion of the images on the mask. However, throughput of thesubstrates through the imaging process tends to be quite low when such amask is utilized, because the smaller block of image patterns for agiven layer must be exposed and repeated multiple times across thesurface of the substrate in order to completely image the substrate withthe block of patterns.

What is needed, therefore, is a system whereby the strengths of variousmask designs can be combined to both increase the efficiency of theexposure process and decrease the costs associated with masks.

SUMMARY

The above and other needs are met by an improvement to a method offabricating a plurality of integrated circuits on a substrate accordingto a first integrated circuit design. Each of the integrated circuits isformed with a plurality of layer patterns. At least one first layerpattern of the layer patterns is common with a second integrated circuitdesign, and at least one second layer pattern of the layer patterns isunique to the first integrated circuit design. The first layer patternis imaged on the substrate using an exposure tool and a first maskhaving a first number of the first layer patterns formed in a blockthereon. No other layer patterns of the first layer patterns and thesecond layer patterns are formed on the first mask. The first number isless than the plurality of integrated circuits formed on the substrate.The first layer patterns are imaged on the substrate by stepping andrepeating the block of first number of first layer patterns across thesubstrate with the exposure tool. The first layer patterns on the firstmask are formed at a size that is larger than a size at which the firstlayer patterns are imaged on the substrate.

The second layer patterns are imaged on the substrate using an exposuretool and a second mask having a second number of the second layerpatterns formed in a block thereon. At least one additional layerpattern of the second layer patterns is formed on the second mask. Thesecond number is less than the plurality of integrated circuits formedon the substrate. The second layer patterns are imaged on the substrateby exposing and repeating the block of second number of second layerpatterns across the substrate with the exposure tool. The second layerpatterns on the second mask are formed at a size that is larger than asize at which the second layer patterns are imaged on the substrate.

In this manner, a single block of a layer pattern that is common acrossmany different integrated circuit designs can be formed on a singlemask, which is then used in the fabrication process for all of thedifferent integrated circuit designs to which it is associated. Thus,separate masks for these common layer patterns are not required, andfewer masks are required. In addition, this block of common layerpatterns can be formed using a relatively large number of images, whichcan be exposed using a relatively wider field in the exposure tool, thusincreasing exposure efficiency. However, for layer patterns that areunique to a given integrated circuit design, the block of unique layerpatterns can be formed on a separate mask with other unique layerpatterns for the same integrated circuit design. Thus, fewer masks arerequired for the unique layer patterns. By forming these block patternswith a relatively smaller number of images, more and different uniqueblock patterns can fit on a single mask, reducing the overall maskcount. These smaller unique block patterns can then be exposed using arelatively narrower field in the exposure tool, so that only the desiredunique block pattern on the mask is exposed.

In various preferred embodiments, the first number of first layerpatterns is greater than the second number of second layer patterns. Theblock of first number of first layer patterns is preferably imaged usinga wide field mode on the exposure tool, and the block of second numberof second layer patterns is imaged using a narrow field mode on theexposure tool. Preferably, the second mask includes separate blocks ofall second layer patterns required for the fabrication of the pluralityof integrated circuits according to the first integrated circuit design.

According to another aspect of the invention there is described a maskset for a first integrated circuit design. The mask set includes a firstmask having only a block of a first number of first layer patterns ofthe first integrated circuit design, where the first number is less thanthe plurality of integrated circuits formed on the substrate. The firstlayer patterns on the first mask are formed at a size that is largerthan a size at which the first layer patterns are imaged on a substrate.The first layer patterns are common between the first integrated circuitdesign and at least one other integrated circuit design.

A second mask has a block of a second number of second layer patterns ofthe first integrated circuit design, where the second number is lessthan the plurality of integrated circuits formed on the substrate. Thesecond layer patterns on the second mask are formed at a size that islarger than a size at which the second layer patterns are imaged on thesubstrate. The second layer patterns are unique to the first integratedcircuit design. The second mask also has a block of a third number ofthird layer patterns of the first integrated circuit design, where thethird number is less than the plurality of integrated circuits formed onthe substrate. The third layer patterns on the second mask are formed ata size that is larger than a size at which the third layer patterns areimaged on the substrate. The third layer patterns are unique to thefirst integrated circuit design.

According to yet another aspect of the invention, there is described amethod of imaging a family of related integrated circuit designs. Thefamily of related integrated circuit designs includes at least a firstlayer pattern in common between the family of related integrated circuitdesigns. Each of the integrated circuit designs within the family ofrelated integrated circuit designs also includes a least a second layerpattern that is unique to a given one of the integrated circuit designswithin the family of related integrated circuit designs.

The first layer pattern is imaged on a substrate using an exposure tooland a first mask having a first number of the first layer patternsformed in a block thereon. No other layer patterns of the first layerpatterns and the second layer patterns are formed on the first mask. Thefirst number is less than a number of integrated circuits formed on thesubstrate. The first layer patterns are imaged on the substrate bystepping and repeating the block of first number of first layer patternsacross the substrate with the stepper. The first layer patterns on thefirst mask are formed at a size that is larger than a size at which thefirst layer patterns are imaged on the substrate

The second layer pattern is imaged on the substrate using an exposuretool and a second mask having a second number of the second layerpatterns formed in a block thereon. The second number is less than thefirst number, an at least one additional layer pattern of the secondlayer patterns is formed on the second mask. The second number is alsoless than the number of integrated circuits formed on the substrate. Thesecond layer patterns are imaged on the substrate by exposing andrepeating the block of second number of second layer patterns across thesubstrate with the exposure tool. The second layer patterns on thesecond mask are formed at a size that is larger than a size at which thesecond layer patterns are imaged on the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantages of the invention are apparent by reference to thedetailed description when considered in conjunction with the figures,which are not to scale so as to more clearly show the details, whereinlike reference numbers indicate like elements throughout the severalviews, and wherein:

FIG. 1 is a top plan view of a first mask having only a single block ofa single layer pattern,

FIG. 2 is a top plan view of a second mask having multiple blocks ofdifferent layer patterns,

FIG. 3 depicts an exposure pattern on a substrate for a smallest uniquelayer pattern, and

FIG. 4 depicts an exposure pattern on a substrate for a larger multipleof the smallest unique layer pattern.

DETAILED DESCRIPTION

With reference now to FIG. 1, there is depicted a top plan view of afirst mask 10 having only a single block 14 of a single layer pattern.The block 14 preferably makes as much use of the surface area of themask 10, and leaves only a small border 12 unused at the peripheraledges of the mask 10. The block 14 represents multiple instances of animage required to form a single layer of an integrated circuit design.Most preferably, this layer is one that is used by more than oneintegrated circuit design, and thus the mask 10 can be used as a part ofthe fabrication process for many different integrated circuit designs.

As depicted in FIG. 1, the block of images 14 is formed to be relativelylarge, so that the exposure tool, most preferably a stepper or ascanner, can be operated in a wide field or full field mode, and many ofthe individual images in the block 14 can be projected in a singleexposure step cycle. In this manner, the use of the mask 10 is veryefficient during the exposure process, because many images are projectedonto a substrate in each exposure step cycle. Further, the mask 10 isalso relatively economical, because it can be used for a layer patternthat is common to more than one integrated circuit design. Thus, thebenefits of both efficiency and economy are realized by use of the mask10.

FIG. 2 is a top plan view of a second mask 16 having multiple blocks 14a, 14 b, and 14 c of different layer patterns. The image blocks 14 a, 14b, and 14 c each contain multiple instances of images required for aunique layer pattern within an integrated circuit design. Thus, theimages in the blocks 14 a, 14 b, and 14 c are not used in more than oneintegrated circuit design, but are only used in a single integratedcircuit design. However, it is appreciated that one of each of the threeblocks 14 a, 14 b, and 14 c may be used in associated with threedifferent integrated circuit designs, or the all of the blocks 14 a, 14b, and 14 c may be different unique layers of the same integratedcircuit design. The blocks 14 a, 14 b, and 14 c are each exposedindividually, with the exposure field of the exposure tool reduced so asto exclude the other blocks, in a narrow field or small field mode.

Because the size of each of the blocks 14 a, 14 b, and 14 c isrelatively small in comparison to the block 14 depicted in FIG. 1, theexposure efficiencies are somewhat lost. However, the economy of onlyusing a single mask for three different image blocks 14 a, 14 b, and 14c reduces the overall costs. Preferably, there is some balance betweenthe size of each of the blocks 14 a, 14 b, and 14 c and the number ofsuch blocks 14 a, 14 b, and 14 c that are placed on the mask 16.

For example, if the mask cost is relatively high, but the cost of havingto make multiple exposures to cover the surface of a substrate with theimages in a block of images 14 is relatively low, then preferably theblocks 14 are formed in the mask 16 as small as possible, and as manydifferent blocks 14 as possible are placed on the mask 16, and fewermasks 16 are required. In the limit, just a single device image isplaced within each block 14 according to this economy.

However, if the mask cost is relatively low, but the cost of having tomake multiple exposures to cover the surface of a substrate with theimages in the block of images 14 is relatively high, then preferably theblocks 14 are formed in the mask 16 at least a bit larger, so that fewerexposure steps are required when using one of the blocks 14. In thelimit, just two blocks 14 are placed on the mask 16 according to thiseconomy. In actual implementation, the number of blocks 14 and thenumber of images within each block 14 will preferably be somewhere inbetween the two limits as described, as the economies or other factorsdictate.

Thus, the present invention combines the throughput advantages of fullfield masks on those layers common to a large number of devices, withthe mask cost reduction advantages of small field masks on those layerscustomized for a smaller number of devices. This invention is a methodof implementing such a mixed large field and small field strategy withinthe lithography area. It addresses both the accommodation of multiplefield sizes within a lithography tool job, and also the correspondingrequirements imposed on the reticle layouts.

According to the present invention, some layers of an integrated circuitdesign will be formed using large field images, and other layers of theintegrated circuit design will be formed using small field images. Thus,there are some considerations in the layout of the images on thereticles that must be accounted for, so as to ensure proper alignmentbetween integrated circuit layers, and proper formation of elements suchas test structures in the scribe lines of the substrate, which typicallyrequire the proper alignment of several different layers.

The requirement on reticle layout is fairly straight forward. All largefield images are preferably formed of an integral number of small imagespacked together. The smallest image preferably acts as a building block,from which any and all larger images are constructed. For larger images,this may result in redundant test features within scribe lines, sincethese features are preferably repeated for every small image buildingblock. However, since test features are often constructed with multipleprocessing layers, this technique is preferred to insure that anycombination of large field and small field masks for various layersstill results in the complete construction of all desired test features.This building block methodology helps ensure that all test features arefully formed, regardless of mask strategy.

Within the job of the lithographic exposure tool, which in this case isa stepper or a scanner, the basic cell or field structure—meaning thegrid the tool uses to determine the locations at which to expose animage—is preferably set up to match the dimensions of the smallest ofthe field sizes being used. In addition, substrate layout—meaning theplacement of this grid onto a substrate, which preferably determines howmany intact dice end up on the substrate—is preferably optimized withrespect to this smallest field size.

Once the cell grid has been defined, the images to be exposed aredefined. One image is preferably defined for the smallest field sizeused. The dimensional information for this image preferably matches thedimensions of the cell grid, since the cell grid is preferably set upbased on the smallest field size used. This is depicted in FIG. 3, whereeach of the image fields 22 is exposed. Next, additional images arepreferably defined for any and all larger field sizes used. It isanticipated that there will typically be only two distinct fieldsizes—small and large. However, there is nothing that prevents the useof three or more different field sizes, so long as all the sizes satisfythe constraints on mask layout, which are described hereafter. Thedimensional information for a larger field size preferably correspondsto the dimensions of the pattern on the full field or large field mask.

The next step in setting up the job is to distribute the images, asdefined above, with respect to the cell grid. The smallest image definedabove is preferably distributed to all cells on the substrate, asdepicted in FIG. 3. In other words, the smallest image is preferablyexposed in every cell of the cell grid. Larger images are preferablydistributed to only some of the cells in the cell grid, as depicted inFIG. 4. The cells 28 to which these larger images 26 are distributed arepreferably chosen such that, after factoring in the actual size of thelarger images, exposing these images at each of the selected cellspreferably results in complete exposure coverage of the substrate, butno exposures are overlapped one with another. For example, if a largeimage is two times as tall and three times as wide as the smallest imageused, as depicted in FIG. 4, then the larger image is preferablydistributed to only one out of the six cells in the cell grid.

Another consideration in setting up the exposure job is insuring thatthe larger images preferably fall directly on top of the smallest image.If a larger image is an odd numbered multiple of the smallest image inboth X and Y axes, as in a three by three case, then it is relativelystraight forward to ensure such proper alignment. The larger image ispreferably distributed to the cells corresponding to the center buildingblock within the larger image. However, if the larger image is an evennumbered multiple of the smallest image in either axis, as depicted inFIG. 4, then the larger image is preferably shifted one way or anotherwith respect to the cell grid along the even multiple axis. In thiscase, a shift equal to one half the length of the smallest image alongthe axis in question results in proper alignment of the larger image tothe smallest image.

This methodology, when followed for both mask layout and exposure toolrecipe, permits the smooth implementation of a scheme in which differentprocessing layers use different exposure field sizes.

The foregoing description of preferred embodiments for this inventionhave been presented for purposes of illustration and description. Theyare not intended to be exhaustive or to limit the invention to theprecise form disclosed. Obvious modifications or variations are possiblein light of the above teachings. The embodiments are chosen anddescribed in an effort to provide the best illustrations of theprinciples of the invention and its practical application, and tothereby enable one of ordinary skill in the art to utilize the inventionin various embodiments and with various modifications as is suited tothe particular use contemplated. All such modifications and variationsare within the scope of the invention as determined by the appendedclaims when interpreted in accordance with the breadth to which they arefairly, legally, and equitably entitled.

1. In a method of fabricating a plurality of integrated circuits on asubstrate according to a first integrated circuit design, where each ofthe integrated circuits is formed with a plurality of layer patterns,where at least one first layer pattern of the layer patterns is commonwith a second integrated circuit design, and at least one second layerpattern of the layer patterns is unique to the first integrated circuitdesign, the improvement comprising the steps of: imaging the at leastone first layer pattern on the substrate using a first mask having afirst number of first layer patterns according to the first integratedcircuit design formed in a block thereon, with no other layer patternsformed on the first mask, where the first layer patterns are imaged onthe substrate by exposing and repeating the block of first layerpatterns across the substrate according to a first grid withoutoverlapping the block of first layer patterns, and imaging the at leastone second layer pattern on the substrate using a second mask having asecond number of second layer patterns according to the first integratedcircuit design formed in a block thereon, where the second number isless than the first number with at least one additional layer patternformed on the second mask, where the second layer patterns are imaged onthe substrate by exposing and repeating the block of second layerpatterns across the substrate according to a second grid withoutoverlapping the block of second layer patterns, where the first grid andthe second grid are aligned such that each integrated circuit and teststructures in scribe lines between the integrated circuits are properlyformed on the substrate according to the first integrated circuitdesign.
 2. A method of imaging a family of related integrated circuitdesigns, where the family of related integrated circuit designs includesat least a first layer pattern in common within the family of relatedintegrated circuit designs, and each given one of the integrated circuitdesigns within the family of related integrated circuit designs includesat least a second layer pattern that is unique to the given one of theintegrated circuit designs within the family of related integratedcircuit designs, the method comprising the steps of: imaging the atleast a first layer pattern on a substrate using a first mask having afirst number of first layer patterns according to the given one of theintegrated circuit designs formed in a block thereon, with no otherlayer patterns formed on the first mask, were the first layer patternsare imaged on the substrate by exposing and repeating the block of firstlayer patterns across the substrate according to a first grid withoutoverlapping the block of first layer patterns, and imaging the at leasta second layer pattern on the substrate using a second mask having asecond number of second layer patterns according to the given one of theintegrated circuit designs formed in a block thereon, where the secondnumber is less than the first number, with at least one additional layerpattern formed on the second mask, where the second layer patterns areimaged on the substrate by exposing and repeating the block of secondlayer patterns across the substrate according to a second grid withoutoverlapping the block of second layer patterns, where the first grid andthe second grid are aligned such that each integrated circuit and teststructures in scribe lines between the integrated circuits are properlyformed on the substrate according to the given one of the integratedcircuit designs.
 3. A method for forming integrated circuits on asubstrate, the method comprising the steps of: exposing a large fieldpattern of a first layer of entire integrated circuits on the substrateaccording to a first grid without any overlap of the large fieldpattern, exposing a small field pattern of a second layer of entireintegrated circuits on the substrate according to a second grid withoutany overlap of the small field pattern, and aligning the first grid andthe second grid one to another during exposure such that the integratedcircuits and test structures in scribe lines between the integratedcircuits are properly formed on the substrate.